Photoluminescence analysis and photoluminescence imaging are particularly valuable for characterizing semiconductor materials, wafers, epitaxial layers, and devices. In photoluminescence analysis, most measurements to date have been made at a single point on a specimen, especially when the specimen is held at low temperature in a Dewar. Because the signal strength is low, considerable effort has been made to increase the throughput of the grating spectrometer, including a method to shape the beam from the laser (or other light source) at the point of impingement on the specimen so that the illuminated area has the same shape as the entrance slit of the grating monochromator, and when imaged on the entrance slit, slightly overfills it, but provides for very efficient collection of the photoluminescence light produced by the specimen, as described by Gerry Auth in U.S. Pat. No. 4,572,668.
Spectrally-resolved photoluminescence mapping of semiconductor wafers with high spatial resolution has recently been described by Tajima, "Characterization of Semiconductors by Photoluminescence Mapping at Room Temperature", Journal of Crystal Growth 103, 1-7 (1990); by Moore et al, "A Spatially Resolved Spectrally Resolved Photoluminescence Mapping System", Journal of Crystal Growth 103, 21-27 (1990); and in Waterloo Scientific Inc. Application Notes on Photoluminescence #1 (1989) and #2 (1990), Waterloo Scientific Inc., 419 Phillip St., Waterloo, Ont. Canada N2L 3X2. In this application it is known to use apparatuses which are essentially scanning-stage non-confocal laser microscopes, in which the exciting laser wavelength is blocked in the detector path, and the remaining light collected from the specimen is focussed on the entrance slit of a grating spectrometer, which is used to measure the wavelength and intensity of the photoluminescence signal at each specimen position. In this situation, the highest spatial resolution is achieved when the focused spot at the point of impingement on the specimen is as small as possible. The technique of shaping the spot described above reduces the spatial resolution, and is therefore not appropriate when the highest possible spatial resolution is required.
In the field of fluorescence microscopy, the confocal microscopes and techniques presently in use have recently been described in "The Handbook of Biological Confocal Microscopy", IMR Press, Madison, Wiss. (1989), edited by Pawley, and in a review paper by Shotton, "Confocal Scanning Optical Microscopy and its Applications for Biological Specimens", Journal of Cell Science 94, 175-206 (1989). Since there may be more than one source of fluorescence at the focal spot of the confocal microscope, it is important to be able to separate the different wavelengths of the two sources. In addition, a particular fluorescence source may emit different wavelengths, and/or intensities, depending on its local environment, so it is important to be able to map changes in spectra with position in the specimen.
In both photoluminescence and fluorescence, it is known that measurement of lifetimes is important. Photoluminescence or fluorescence decay is usually measured using a pulsed or modulated light source, and the decay of the fluorescence or photoluminescence signal is monitored with a high speed detector. In many cases, more than one lifetime signal is detected, and those signals are mixed together in the detected signal. It is important to be able to separate these lifetimes, and good spatial resolution is also important. In the case of fluorescence measurements, fluorescence recovery after bleaching is also important. In all of these cases, high spectral and spatial resolution in the instrument used to make the measurements, as well as good photon collection efficiency, are important.
A simple prior art confocal scanning laser microscope is shown in FIG. 1. In this implementation the beam from laser 102 is focused by lens 104 on pinhole 106, and the light passing through the pinhole passes through beamsplitter 108 and is focused by objective lens 110 to a focal spot 111 which is diffraction-limited at the surface of (or inside) specimen 112. Light reflected from or emitted by the specimen at focal spot 111 is collected by objective lens 110, and part of this light is reflected by beamsplitter 108 to be focused at detector pinhole 114, which is confocal with focal spot 111 at the specimen and pinhole 106. Light passing through detector pinhole 114 is collected by detector 116. The combination of detector pinhole 114 and detector 116 is a confocal detector. Light from focal spot 111 at specimen 112 passes through detector pinhole 114, but light from any other point on the specimen runs into the edges of detector pinhole 114, and is not collected. Thus, out-of-focus signals are rejected. This gives the confocal microscope the ability to do optical tomography, which allows it to record true three dimensional images. The microscope shown in FIG. 1 uses scanning stages 118 to move the specimen under the stationary laser beam to record the image, but it is also possible to scan the beam instead of scanning the specimen. Microscopes using infinity-corrected optics are also common, both with scanning stages and in scanning-beam configurations. These configurations are described in "The Handbook of Biological Confocal Microscopy" edited by pawley. In addition, detector pinhole 114 and detector 116 behind it can be replaced by a small detector whose area is the same as that of detector pinhole 114.
Confocal Scanning Laser Microscopes have been used to record photoluminescence and fluorescence images with high spatial resolution (in three dimensions) using filters to block the exciting wavelength, but accepting all (or substantially all) wavelengths of the luminescence signal. One possible prior art configuration for such a microscope is shown in FIG. 2, where dichroic beamsplitter 200 transmits light at the wavelength of the incoming laser beam but reflects most of the longer wavelength photoluminescence or fluorescence emitted from specimen 112 towards detector pinhole 114. To further reduce the small amount of reflected laser light, blocking filter 202 which blocks light at the laser wavelength can be placed in the detection arm of the microscope, as shown.
Three known implementations of a confocal microscope that can measure spectrally-resolved data are as follows. It is known that a bandpass filter can be placed in the detection arm of the microscope, either in front of or behind the detector pinhole as described by Stelzer in "Considerations on the intermediate optical system in confocal microscopes", a chapter in "The Handbook of Biological Confocal Microscopy", edited by Pawley. This may allow the operator to separate the emission bands of two fluorophores by using two different bandpass filters, or to measure a crude spectrum by changing filters each time a new wavelength is to be measured, but this technique is impractical for measuring a complete spectrum with good spectral resolution.
A second known implementation is to focus the light emitted from the detector pinhole of a confocal photoluminescence or fluorescence microscope onto the entrance slit of a grating monochromator (or to place the monochromator in a position such that it's entrance slit replaces the detector pinhole). These solutions both pas to the detector only a fraction of the photoluminescence light collected by the microscope, and are expensive because a complete grating monochromator is required.
A third known implementation uses a lens to focus light from the detector pinhole onto the input aperture of a Fourier Transform Infrared Spectrometer, or any other type of spectrometer that is appropriate for the wavelength range involved.
Presently, the simplest confocal fluorescence microscopes usually use a dichroic beamsplitter to separate the longer fluorescence wavelengths from the exciting wavelength, and detect all of the fluorescence wavelengths at once (one such microscope is shown in FIG. 2). This implementation does not allow the operator to make spectrally-resolved measurements.
The present non-confocal photoluminescence mapping system sold by Waterloo Scientific Inc. works by focusing the light from the non-confocal microscope onto the entrance slit of a grating monochromator, which is expensive, and all of the light collected by the objective lens does not reach the grating of the monochromator.
An object of this invention is to provide a scanning microscope or mapping system that has both good spatial resolution and good spectral resolution, and at the same time is very efficient in collecting light emitted from the specimen.
A further object of this invention is to reduce the cost of the spectrally-resolved microscope or mapping system by integrating the spectrally-resolving element into the detection arm of the microscope.